1. Field of the Invention
The present invention relates to a fuel supply control of an engine for automobiles, and in particular, to a method of controlling fuel supply suitable for performing the control to maintain an air-fuel ratio at a proper value.
2. Description of the Related Art
In a prior art fuel supply control system of the feedback control type, a fundamental fuel supply quantity Ti(n) (usually, given by a valve opening time period of a fuel injection valve) in an an n-th stroke is determined based on an air flow rate Q.sub.ay (n-1) at the inlet of a manifold measured in an (n-1)th stroke (n is an integer, and one stroke corresponds to 1/2 revolutions in a 4-cycle engine) and an engine speed N(n-1) as expressed by the following formula ##EQU1## where, k: a correction coefficient.
The determined quantity of fuel is supplied to each cylinder. This fundamental fuel supply quantity Ti(n) is a value when the engine is in a steady state. At the transient time when the throttle valve is opened or closed as during acceleration or deceleration, a correction is made by adding a correction quantity to the fundamental fuel supply quantity. This correction quantity is obtained as a function of the amount of variation .DELTA..theta..sub.th (n-1) with time in the degree of opening of the throttle valve as expressed by the following formula EQU k=1+func(.DELTA..theta..sub.th (n-1)) (2),
and the fuel quantity to be supplied is determined by correcting the Ti(n) in formula (1) by the correction quantity k.
The calculation method according the formula (1) is to be determined by the fuel quantity to be supplied in the next n-th stroke by using the measured values including the air flow rate and engine speed in the (n-1)th stroke. In this method, if the intake air flow rate or engine speed is changed to a great extent between the (n-1)th stroke and the n-th stroke, the fuel quantity supplied in the n-th stroke will be deviated from a required fuel quantity in the n-th stroke. Thus, the A/F ratio (air to fuel ratio) will also be deviated from a target value. The appropriate fuel quantity to be supplied should be a value which matches the amount of air actually flowing into each cylinder in the n-th stroke. However, this amount of air flowing into the cylinder cannot be measured by the technique at the present time. Even it the amount of air flow into the cylinder can be measured, since a delay is involved in the calculation, it results in that the present fuel quantity is calculated based on the amount of air in the past stroke. For this reason, at the transient time, since a significant error is caused in the air-fuel ratio control, it is necessary to design the exhaust gas control device (catalyst, EGR, etc.) with a sufficient margin in the characteristic thereof more than required. Thus, there has been a problem in the cost and the drivability.
The formula (2) is intended to compensate for a follow-up delay in the fuel supply quantity during a transient state by using a change in the degree of opening of the throttle valve. Practically, however, much time and labor have been spent to experimentally obtain a function of the correction coefficient which satisfied both the reduction and exhaust gas components an the drivability. Although, not less than 50% of the development period of the control logic has been devoted, there is a problem in that the accuracy of control of the air-fuel ratio is still low.